The 'stepping stone' rule. Left: the initial
condition: 256 colors distributed randomly. Right: the result
of applying the rule repeatedly.

The rule operates as follows:
First, choose a number between 0 and 1; this will be the update
probability for all cells. At every time step, generate a random
number between 0 and 1 for every cell. If the number is lower
than the update probability, the color of the cell changes to
that of one of its neighbors (selected uniformly at random).
(The neighbors are defined here as the four orthogonally adjacent
cells: north, east, south, west.)